Exposure to HIV-infected semen accounts for most viral transmissions worldwide, however the factors that determine the HIV viral load and infectivity of semen are not well understood. We have recently discovered a new type of amyloid fibril in the semen, derived from the semenogelins, which enhances the ability of HIV to infect target cells. These fibrils act similarly to semen enhancer of viral infectivity (SEVI), the first amyloid fibril identified in semen. Strikingly we have found that the levels of semenogelins in semen differ markedly between HIV infected men, and that levels correlate directly with the semen HIV viral load, independent of the blood HIV viral load. Conversely, semen viral load did not correlate with levels of SEVI or its protein precursor, prostatic acid phosphatase (PAP). We hypothesize that the semen HIV viral load is driven in part by the interaction of HIV with these semenogelin amyloid fibrils produced within the seminal vesicles. If correct, this model could explain the marked variability in semen HIV viral load and infectivity that is observed among HIV-infected men. These findings could also identify novel targets for biomedical interventions to greatly reduce male-female and male-male transmission of HIV infection. We propose to conduct a series of clinical and translational studies to test and refine our hypothesis that semenogelin- derived amyloid fibrils influence HIV viral load and infectivity in semen. In Specifi Aim 1, we will further investigate the biology of the semenogelins and semenogelin-derived amyloid fibrils using seminal vesicle tissue obtained from patients undergoing urologic surgery. We will assess semenogelin amyloid fibril levels and HIV enhancing activity of seminal vesicle fluid from these patients, and determine whether specific factors (such as PSA levels, or inflammatory changes in the tissue) may influence amyloid fibril expression. In Specific Aim 2, we will perform both cross-sectional and longitudinal studies with HIV-infected semen donors to determine whether seminal vesicle-derived amyloid fibrils (promoting infection of target cells) and genital inflammation (increasing target cell availability) influence the semen HIV viral load. In Specific Aim 3, we will examine whether semen amyloid fibril levels affect HIV transmission risk using a case-control study approach with HIV-infected men. Specifically we will compare semen amyloid levels between men who have transmitted HIV to their sexual partners (transmitters) with those from other men who also had partners exposed to their semen, but whose partners remained uninfected (non-transmitters). At the completion of this project, we will have tested the predictions of a new model of HIV infectivity and HIV transmission based on host-derived amyloid fibrils present in seminal vesicles. By refining our fundamental understanding of HIV transmission biology including the role of novel host factors, we hope to propel new approaches for preventing HIV transmission that can be used synergistically with antiretroviral therapy.